Tuesday 27 November 2007 19.17 EST
First published on Tuesday 27 November 2007 19.17 EST

One reason Gordon Brown gave for not holding an election was to have time to roll out his vision. It is not a meaning of the word that Britain's greatest revolutionary poet would have recognised; William Blake, born 250 years ago today, had what George Bush Sr called "the vision thing" in the way other people have headaches or fits of laughter. At four he glimpsed God's head at the window, at eight a tree shimmering with angels. For Blake, being a visionary meant seeing beyond a version of politics centred chiefly on parliament. "House of Commons and House of Lords seem to me to be fools," he wrote. "They seem to me to be something other than human life."

Like Brown, Blake grew up in a lower-middle-class Christian milieu. But the culture from which Blake sprang was one of the most precious Britain has produced, in which Jacobin artisans and Republican booksellers rubbed shoulders with Dissenting preachers and occult philosophers; the country was effectively a police state, ridden with spies and hunger rioters. Brown's Britain is not yet a police state, but its technologies of spying and surveillance surpass the wildest dreams of the autocrats of Blake's day. Blake himself was tried for sedition and acquitted, having allegedly cried in public: "Damn the king and his country!" Today whole sectors of the labour movement bow the knee to monarchy, or at least tolerate it as a minor irritant. The history of labour from Blake to Brown is, among other things, how dissent became domesticated.

Blake's politics were not just a matter of wishful thinking, as so many radical schemes are today. Across the Atlantic one great anti-colonial revolution had held out the promise of liberty, and to the poet's delight another had broken out in the streets of Paris. Together they promised to bring an end to the rule of state and church - "the Beast and the Whore", as Blake knew them. Most of our own writers, however, seem to know little of politics beyond the value of individual liberties.

In this, they are faithful to the libertarian lineage of John Milton; but Milton knew rather more about politics than freedom of expression. In his greatest poem, he mourned the paradise that radical Puritans had hoped to witness on earth. As mythologer-in-chief of the English 17th-century revolution, he urged the cutting off of the king's head, and was lucky to escape with his own. It is hard to imagine Craig Raine or Ian McEwan posing a threat to the state.

In his own mighty epic - Milton - Blake turned back to his great Protestant forebear from a Britain now scarred by industrial capitalism. He raided Milton's work to foster his own visions of liberation, passing on the revolutionary torch to WB Yeats. This self-appointed mythmaker to the Irish war of independence was inspired by Blake's notion of the poet as prophet and public activist.

Politics today is largely a question of management and administration. Blake, by contrast, viewed the political as inseparable from art, ethics, sexuality and the imagination. It was about the emancipation of desire, not its manipulation. Desire for him was an infinite delight, and his whole project was to rescue it from the repressive regime of priests and kings. His sense of how sexuality can turn pathological through repression is strikingly close to Freud's. To see the body as it really is, free from illusion and ideology, is to see that its roots run down to eternity. "If the doors of perception were cleansed," he claims, "everything would appear to man as it is, infinite." Political states keep power by convincing us of our limitations.

They do so, too, by persuading us to be "moderate"; Blake, however, was not enamoured of the third way. The New Testament that Gordon Brown reads in his Presbyterian fashion as a model of prudence, conscience and sobriety, Blake read as a hymn to creative recklessness. He sees that Jesus's ethics are extravagant, hostile to the calculative spirit of the utilitarians. If they ask for your coat, give them your cloak; if they ask you to walk one mile, walk two. The road of excess leads to the palace of wisdom, and those who restrain their desires do so because their desires are feeble enough to be restrained.

The energy captured in Blake's watercolours and engravings is his riposte to mechanistic thought. In a land of dark Satanic mills, the exuberant uselessness of art was a scandal to hard-headed pragmatists. Art set its face against abstraction and calculation: "To generalise is to be an Idiot," Blake writes. And again: "The whole business of Man is the arts, and all things in common." The middle-class Anglicans who sing his great hymn Jerusalem are unwittingly celebrating a communist future.

Brothels, Blake wrote, are built with bricks of religion. Today, hardly a single Christian politician believes with Blake that any form of Christian faith that is not an affront to the state is worthless. Blake was no dewy-eyed radical, convinced as he was of the reality of the Fall. He had a radical Protestant sense of human corruption. His vision of humankind was darker than that of the Panglossian progressives of our own time, with their vacuous talk of "moving on". Yet it was more hopeful as well. London had lapsed into Babylon; but it remained true that "everything that lives is holy", and it might still prove possible to transform the city into the New Jerusalem.

· Terry Eagleton is John Edward Taylor professor of English literature at Manchester University comment@theguardian.com